The Sumatran orangutan (Pongo abelii) is one of the two species of orangutans. Found only on the island of Sumatra in Indonesia, it is rarer than the Bornean orangutan. Its common name is based on two separate local words, "orang" ("people" or "person") and "hutan" ("forest"), and translates as 'man of the forest'.

Wild Sumatran orangutans in the Suaq Balimbing swamp have been observed using tools.[5] An orangutan will break off a tree branch that is about a foot long, snap off the twigs and fray one end. With its teeth,[6] the orangutan will use the stick to dig in tree holes for termites. They will also use the stick to poke a bee's nest wall, move it around and catch the honey. In addition, orangutans use tools to eat fruit. When the fruit of the Neesia tree ripens, its hard, ridged husk softens until it falls open. Inside are seeds that the orangutans enjoy eating, but they are surrounded by fiberglass-like hairs that are painful if eaten. Tools are created differently for different uses. Sticks are often made longer or shorter depending on whether they will be used for insects or fruits.[6] If a particular tool proves useful, the orangutan will often save it. Over time they will collect entire "toolboxes".[6] A Neesia-eating orangutan will select a five-inch stick, strip off its bark, and then carefully collect the hairs with it. Once the fruit is safe, the ape will eat the seeds using the stick or its fingers. Although similar swamps can be found in Borneo, wild Bornean orangutans have not been seen using these types of tools.

NHNZ filmed the Sumatran orangutan for its show Wild Asia: In the Realm of the Red Ape; it showed one of them using a simple tool, a twig, to pry food from difficult places. There is also a sequence of an animal using a large leaf as an umbrella in a tropical rainstorm.

As well as being used as tools, tree branches are a means of transportation for the Sumatran orangutan. The orangutans are the heaviest mammals to travel by tree. This makes them particularly susceptible to the changes in arboreal compliance. To deal with this their locomotion is characterized by the slow movement, long contact times, and an impressively large array of locomotors postures. Orangutans have even been shown to utilize the compliance in vertical supports to lower the cost of locomotion by swaying trees back and forth and they possess unique strategies of locomotion moving slowly and using multiple supports to limit oscillations in compliant branches, particularly at their tips.[citation needed]

The Sumatran orangutan is also more arboreal than its Bornean cousin; this could be because of the presence of large predators like the Sumatran tiger. It moves through the trees by quadrumanous locomotion and semibrachiation.[citation needed]

As of 2015, the Sumatran orangutans species only has approximately 7000 remaining members in its population. The World Wide Fund for Nature is thus carrying out attempts to protect the species by allowing them to reproduce in the safe environment of captivity. However, this comes at a risk to the Sumatran orangutan’s native behaviors in the wild. While in captivity, the orangutans are at risk to the "Captivity Effect": animals held in captivity for a prolonged period will no longer know how to naturally behave in the wild. Being provided with water, food, and shelter while in captivity and lacking all the challenges of living in the wild, captive behaviour becomes more exploratory in nature.[7]

A repertoire of 64 different gestures in use by orangutans has been identified. 29 of these are thought to have a specific meaning that can be interpreted by other orangutans the majority of the time. 6 intentional meanings were identified: Affiliate/Play, Stop action, Look at/Take object, Share food/object, Co-locomote and Move away. Sumatran orangutans do not use sounds as part of their communication, which includes a lack of audible danger signals, but rather base their communication on gestures alone.[8]

The Sumatran orangutan has five stages of life that are characterized by different physical and behavioral features. The first of these stages is infancy which last from birth to around 2.5 years-old. The orangutan weighs between 2 and 6 kilograms. An infant is identified by light pigmented zones around the eyes and muzzle in contrast to darker pigmentation on the rest of the face as well as long hairs that protrude outward around the face. During this time the infant is always carried by the mother during travel, she/he is highly dependent on the mother for food, and also sleeps in the mother's nest. The next stage is called juvenile-hood and takes place between 2.5 and 5 years-old. The orangutan weighs between 6 and 15 kilograms and does not look dramatically different from an infant. Although s/he is still mainly carried by the mother, a juvenile will often play with peers and make small exploratory trips within the vision of the mother. Toward the end of this stage, the orangutan will stop sleeping in the mother's nest and will build its own nest nearby. From the ages of 5 to 8 years-old, the orangutan is in an adolescent stage of life. S/he weighs around 15-30 kilograms. The light patches on the face start to disappear and eventually the face becomes completely dark. During this time orangutans still have constant contact with their mothers, yet they develop a stronger relationship with peers while playing in groups. They are still young and act with caution around unfamiliar adults, especially males. At 8 years-old, female orangutans are considered fully developed and begin to have offspring of their own. Males, however, enter a stage called sub-adulthood. This stage lasts from 8 to around 13 or 15 years-old and the orangutans weigh around 30 to 50 kilograms. Their faces are completely dark and they begin to develop cheek flanges. Their beard starts to emerge while the hair around their face shortens and instead of pointing outwards it flattens along the skull. This stage marks sexual maturity in males, yet these orangutans are still socially undeveloped and will still avoid contact with adult males. Finally, male Sumatrans orangutans reach adulthood at 13 to 15 years of age. They are extremely large animals weighing between 50 and 90 kilograms, roughly the weight of a fully grown human. They have a fully grown beard, fully developed cheek callosities, and long hair. These orangutans have reached full sexual and social maturity and now only travel alone.[9]

The Sumatran orangutan is more social than its Bornean counterpart; groups gather to feed on the mass amounts of fruit on fig trees. The Sumatran orangutan community is best described as loose, not showing social or spatial exclusivity. Groups generally consist of female clusters and a preferred male mate. However, adult males generally avoid contact with other adult males. Subadult males will try to mate with any female, although mostly unsuccessfully, since mature females are easily capable of fending them off. Mature females prefer to mate with mature males. Usually, there is a specific male in a group that mature females will exhibit preference for.[10] Male Sumatran orangutans sometimes have a delay of many years in the development of secondary sexual characteristics, such as cheek flanges and muscle mass.[11]

Males exhibit bimaturism, whereby fully flanged adult males and the smaller unflanged males are both capable of reproducing, but employ differing mating strategies to do so.[1]

The average interbirth rates for the Sumatran orangutan is 9.3 years, the longest reported among the great apes, including the Bornean orangutan. Infant orangutans will stay close to their mothers for up to three years. Even after that, the young will still associate with their mothers. Both orangutan species are likely to live several decades; estimated longevity is more than 50 years. The average of the first reproduction of P. abelii is around 15.4 years old. There is no indication of menopause.[3]

Nonja, thought to be the world's oldest orangutan in captivity or the wild at the time of her death, died at the Miami MetroZoo at the age of 55.[12]

Sumatran orangutans are primarily frugivores, favoring fruits consisting of a large seed and surrounded by a fleshy substance, such as fig fruits.[13][14] Insects are also a huge part of the orangutan's diet; the most consumed types are ants, predominantly of the genus Camponotus (at least four species indet.).[14] Their main diet can be broken up into five categories: fruits, insects, leaf material, bark and other miscellaneous food items. Studies have shown that orangutans in the Ketambe area in Indonesia ate over 92 different kinds of fruit, 13 different kinds of leaves, 22 sorts of other vegetable material such as top-sprouts, and pseudo-bulbs of orchids. Insects included in the diet are numbered at least 17 different types. Occasionally soil from termite mounds were ingested in small quantities.[14] When there is low ripe fruit availability, Sumatran orangutans will eat the meat of the slow loris, a nocturnal primate. Water consumption for the orangutans was ingested from natural bowls created in the trees they lived around. They even drank water from the hair on their arms when rainfall was heavy.[15]

Meat-eating happens rarely in Sumatran orangutan, and orangutans do not show a male bias in meat-eating. A research in Ketambe area reported cases of meat-eating in wild Sumatran orangutans, of which 9 cases of orangutans eating slow lorises. The research shows, in the recent 3 cases of slow lorises eaten by Sumatran orangutan, a maximum mean feeding rate of the adult orangutan for an entire adult male slow loris is 160.9 g/h and, of the infant, 142.4 g/h. No case have been reported during mast years, which suggests orangutans take meat as a fallback for the seasonal shortage of fruits; preying on slow loris occurs more often in periods of low fruit availability. Similar to most primate species, orangutans appear to only share meat between mother and infants.[15]

The researchers also published less complete copies from 10 wild orangutans, five from Borneo and five from Sumatra. The genetic diversity was found to be lower in Bornean orangutans (Pongo pygmaeus) than in Sumatran ones (Pongo abelii), despite the fact that Borneo is home to six or seven times as many orangutans as Sumatra. The comparison has shown these two species diverged around 400,000 years ago, more recently than was previously thought. The orangutan genome also has fewer rearrangements than the chimpanzee/human lineage.[17] The full sequence and annotation can be viewed on the Ensembl Genome Browser.

Sumatrans encounter threats such as logging (both legal and illegal), wholesale conversion of forest to agricultural land and oil palm plantations, and fragmentation by roads. Oil companies use a method of deforestation to utilize palm oil. This palm oil is taken from the trees in which Sumatran orangutans live and swing from. An assessment of forest loss in the 1990s concluded that forests supporting at least 1,000 orangutans were lost each year within the Leuser Ecosystem alone.[1]

While poaching generally is not a huge problem for the Sumatrans, occasional local hunting does decrease the population size. They have been hunted in the Northern Sumatra in the past as targets for food; although deliberate attempts to hunt the Sumatrans are rare nowadays, locals such as the Batak people are known to eat almost all vertebrates in their area. Additionally, the Sumatrans are treated as pests by Sumatran farmers, becoming targets of elimination if they are seen damaging or stealing crops. For commercial aspects, hunts for both dead or alive specimens have also been recorded as an effect of the demand by European and North American zoos and institutions throughout the 20th century.[14]

Sumatran orangutans have developed a highly functioning cardiovascular system. However, with this development air sacculitis has become more prevalent among orangutans in this species, due to the new hugely improved air sacs in their lungs. Air sacculitis is similar to Streptococcus i.e. strep throat in Homo sapiens. The bacterial infection is becoming increasing common in captive orangutans, due to the fact that captive orangutans are exposed to the human strain of Streptococcus in captivity. At first, both strains are treated and cured with antibiotics along with rest. Yet, in 2014 a Sumatran orangutan, ten years in captivity was the first of its species to die from Streptococcus anginosus. This remains the only known case, but raises the question of why the known human cure for Streptococcus was ineffective in this case.[20]

A survey published in March 2016 estimates a population of 14,613 Sumatran orangutans in the wild, doubling previous population estimates.[23] A survey in 2004 estimated that around 7,300 Sumatran orangutans still live in the wild. The same study estimates a 20,552 km2 occupied area for the Sumatran orangutans, of which only an approximate area range of 8,992 km2 harbors permanent populations.[13] Some of them are being protected in five areas in Gunung Leuser National Park; others live in unprotected areas: northwest and northeast Aceh block, West Batang Toru river, East Sarulla and Sidiangkat. A successful breeding program has been established in Bukit Tiga Puluh National Park in Jambi and Riau provinces. The main reason for the endangerment of these orangutans is because of palm oil companies destroying the native rain forests.

Two strategies that are recently being considered to conserve this species are 1) rehabilitation and reintroduction of ex-captive or displaced individuals and 2) the protection of their forest habitat by preventing threats such as deforestation and hunting. The former was determined to be more cost efficient for maintaining the wild orangutan populations, but comes with longer time scale of 10–20 years. The latter approach has better prospects for ensuring long-term stability of populations.[24] This type of habitat conservation approach has been pursued by the World Wide Fund for Nature, who joined forces with several other organizations to stop the clearing of the biggest part of remaining natural forest close to the Bukit Tigapuluh National Park.[15]

In addition to the above extant wild populations, a new population is being established in the Bukit Tigapuluh National Park (Jambi and Riau Provinces) via the re-introduction of confiscated illegal pets. This population currently numbers around 70 individuals and is reproducing.[1] However it has been concluded that forest conservation costs twelve times less than reintroducing orangutans into the wild, and conserves more biological diversity.[24]

Orangutans have large home ranges and low population densities, which complicates conservation efforts. Population densities depend to a large degree on the abundance of fruits with soft pulp. Sumatran orangutan will commute seasonally between lowland, intermediate, and highland regions, following fruit availability. Undisturbed forests with broader altitudinal range can thus sustain larger orangutan populations; conversely, the fragmentation and extensive clearance of forest ranges breaks up this seasonal movement. Sumatra currently has one of the highest deforestation rates in the world.[25]